Typically, caches are small, fast storage buffers employable to store information, such as instruction code or data, in order for a processing device to more quickly and efficiently have access to the information. Typically, it is faster for the processing device to read the smaller memory of the cache than to read a main memory. Also, with the rapid increase of intensive computational requirements, such as graphical processing and advanced numerical analysis, their importance in a computing system will only increase.
A cache is a limited resource. One way to allocate sets of a cache is to employ a hardware least recently used (LRU) function to determine replacement of sets. There are other hardware replacement algorithms including most recently used and first in first out. Given the LRU information, the cache determines the last set accessed with the cache in the case of a cache miss. The cache replaces the LRU information in the event of a cache miss, that is, the cache did not have the requested information. This replacement allows the processor to quickly access the selected new information the next time this information is selected. Furthermore, this replacement also increases the chances of the processor finding associated information, as the replaced set cache data is likely to have temporal or spatial locality.
However, there are issues related to the employment of a hardware LRU function in a cache. For instance, some information, such as streaming data, tends to replace large amounts of useful instructions or data in a cache. It would be desirable to isolate this replacement to certain sections of the cache, and leave other sections to be used opportunistically. Furthermore, there are certain critical data and instructions which will be replaced in the cache by the normal LRU. It would be desirable to lock these in the cache and not allow the LRU to replace them. Therefore, what is needed is a cache management scheme which overcomes these limitations.